1. Field of the Invention
The invention disclosed herein relates generally to configuration and performance of the semiconductor power devices and circuits. More particularly, this invention relates to circuit design, device configuration and manufacturing method for providing thermally stable semiconductor power devices such as the trenched MOSFET device.
2. Description of the Prior Art
The technical challenges of maintaining thermal stability of the semiconductor power devices become a special concern with the advent of high-speed metal oxide semiconductor (MOS) gate devices. Particularly, when the semiconductor power devices such as a MOSFET device is implemented for power-switching applications, the operation often subject the device to operate under a high current and high voltage condition. These operation conditions lead to great power dissipation and causes temperature to rise rapidly. As will be further discussed below, unless properly controlled, the semiconductor power device can encounter a thermal runaway phenomenon that would eventually lead to device and system failures.
There is an urgent demand to resolve such technical problems because the semiconductor power devices such as the MOSFET transistors are widely implemented in wide varieties of electronic systems and particularly applied for high power and high frequency switching operations. When the MOSFET transistors are used in applications such as the low dropout voltage regulator (LDO), the power MOSFET transistors are operated in the saturated region where both the source to drain voltage, i.e., Vds, and the source to drain current, i.e., Ids, are high simultaneously. Under this operating condition, the power dissipation of the MOSFET will cause the temperature to rise. If designed improperly, thermal runaway will occur and the Power MOSFET transistor will fail due to overheating.
FIG. 1 depicts a typical drain current versus the applied gate-to-source voltage, i.e., Vgs, of a Power MOSFET transistor for a fixed source to drain voltage Vds. As shown in FIG. 1, there is no appreciable drain current, i.e., Ids, until the applied Vgs exceeds the threshold voltage of the transistor. When the operating temperature of the Power MOSFET transistor is increased, the threshold voltage (Vt) of the transistor will decrease due to a negative temperature coefficient (TC). With the increase of temperature, the gate overdrive voltage, as defined by (Vgs−Vt) where Vt is the threshold voltage, will also increase. This higher gate overdrive voltage will further increase the drain current of the transistor thus causes a further increase in the operating temperature. In the meantime, in response to a higher operating temperature, the drain current of the transistor tends to decrease because of the decrease of the mobility of the carrier with rising temperature. Therefore, there are two competing effects caused by an increase in the operating temperature. On the one hand, the higher temperature leads to lower threshold voltage that causes a higher current while on the other hand, the higher temperature causes a decrease in carrier mobility that leads to a lower current. Unfortunate these two competing and conflict effects do not exactly cancel each other. FIG. 2 illustrates the shift of the threshold voltage Vt by superimposing the Ids versus Vgs curves at approximately room temperature of 25 degrees Celsius and 125 degrees Celsius. On the left side of these two curves is a region of negative temperature coefficient and on the right is a region of positive temperature coefficient.
Referring to FIG. 2 again, the reduction in threshold is indicated by the shift of the knee of the Ids curve to the left. The reduction in carrier mobility reduces the slope of the Ids curve. The crossover point of the two curves is commonly defined as the Ids0. The temperature coefficient (TC) of the drain current is negative if the operating current of the MOSFET is larger than Ids0. The temperature coefficient will be positive if the operating current of the MOSFET is lower than Ids0. A thermal runaway occurs when the operating current and voltage of the MOSFET transistor cause the temperature to rise and the higher temperature causes the current to increase and further increase the temperature of the transistor. This problem is even more serious for trench Power MOSFET transistor due to its higher trans-conductance.
There have been numerous approaches proposed to address this problem. A low trans-conductance planar MOSFET is recommended by C. Blake, et al., in “Evaluating the Reliability of Power MOSFETs,” Power Electronics Technology, November, pp. 40-44, 2005. Alternately, different techniques are presented by providing a ballasting the source of the transistor that also helps to reduce the effects of the problem. Both of these two approaches increase the ON resistance of the MOSFET transistor. As a consequence, a lager or more costly transistor is required for implementing such solution to overcome the thermal problems of thermal instability.
Therefore, a need still exists in the art to provide an improved device design and test configurations and methods to overcome the above discussed limitations and difficulties.